1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically include base stations that provide wireless connectivity to a geographical area such as a cell or a sector of a cell. The base stations communicate with mobile units in the cell or sector over an air interface. The air interface supports downlink (or forward link) communication from the base station to the mobile unit and uplink (or reverse link) communication from the mobile unit to the base station. The uplink and downlink communication uses corresponding uplink and downlink channels, which may be defined temporally, in frequency space, spatially, orthogonal codes, or combinations thereof. Examples of standards and are protocols that are used to define uplink and/or downlink channels include Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Multiple Input Multiple Output (MIMO), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA), and the like.
Performance of the uplink channels is typically very sensitive to interference caused by transmissions on other uplink and/or downlink channels. For example, if the uplink channels are not orthogonal to each other, the performance of an uplink channel from a first mobile unit to a base station may be limited by interference caused by transmission on other uplink channel from a second mobile unit to the base station. Conversely, uplink transmissions from the first mobile unit may cause interference that limits the performance of the second mobile unit. Even uplink channels that are orthogonal within a sector are seldom orthogonal across sectors. Thus, uplink transmissions from different mobile units across different sectors to different base stations almost always cause mutual interference. Uplink power control and fractional frequency reuse techniques may be used to reduce interference between non-orthogonal uplink channels. These techniques typically yield modest capacity gains particularly for users near a cell edge. However, they often come at the cost of overall reduction in sector throughput. Orthogonality between the channels can also reduce interference between the orthogonal uplink and/or downlink channels. However, the uplink performance of orthogonal systems such as OFDMA, FDMA, TDMA, and SC-FDMA may still be significantly impacted by interference from uplink transmissions in other sectors or cells.
A base station may also use decoded symbols transmitted over the uplink by one mobile unit in a cell or sector to estimate the interference caused by the mobile unit. The decoded interference signals after reconstructing what was transmitted may then be removed from the received uplink signals of other mobile units in the cell or sector. This technique, which is typically referred to as interference cancellation, may reduce the mutual interference between mobile units within a cell or sector. However, the uplink performance may still be significantly impacted by interference from uplink transmissions in other sectors or cells.